In a photolithography process of semiconductor manufacturing processes, a photomask is used. With the miniaturization of semiconductor devices, a demand for miniaturization in this photolithography process has been increasing. Particularly, an increase in NA of an exposure apparatus using ArF exposure light (193 nm) has proceeded for adaptation to the miniaturization and a further increase in NA is proceeding following the introduction of the immersion exposure technique. For adaptation to the demand for the miniaturization and the increase in NA described above, it is required to enhance the flatness of a photomask. That is, in view of the fact that the allowable amount of position offset of a transfer pattern due to the flatness has been reduced with the reduction in pattern line width and that the focus latitude in the lithography process has been reduced with the increase in NA, the flatness of main surfaces of a mask substrate, particularly the main surface on the side where a pattern is to be formed (hereinafter, the main surface on this side will be referred to simply as a main surface or a substrate main surface), is becoming more important.
On the other hand, when the photomask is chucked on a mask stage of an exposure apparatus by a vacuum chuck, it may happen that the photomask is largely deformed upon chucking due to the affinity with the mask stage or the vacuum chuck. That is, conventionally, since the product management is conducted in terms of the flatness of the photomask before chucking, it may happen that even if the photomask is excellent before chucking, when the photomask is chucked on the mask stage of the exposure apparatus, the flatness thereof is largely degraded depending on the affinity with the mask stage or the vacuum chuck. This tendency is outstanding particularly in the case of a substrate that tends to be distorted due to relatively low symmetry of the shape of its main surface. Thus, it is becoming necessary to consider the flatness of the photomask when it is chucked by the vacuum chuck. There has conventionally been proposed a method of selecting a mask substrate having excellent flatness after chucking on a mask stage of an exposure apparatus (see, e.g. Patent Document 1). It is described that, in this method, it is possible to select a mask substrate having a surface shape with excellent flatness both before and after chucking on a mask stage of an exposure apparatus by measuring the surface shape and the flatness of a main surface of each of a plurality of mask substrates and then simulating a change in flatness when the substrate is chucked on the mask stage.